The present invention relates to electric power plants and more particularly to control systems employed with coal pulverizers in such plants.
In the operation of a coal fired electric power plant, coal is fed to a pulverizer mill where it is finely ground for mixture with the primary air flow and transported to the burners. In many cases, the inlet hot and cold air flow mix is varied by damper positioning to hold a desired mill exit temperature for the primary air and coal mixture. Further, the total primary air flow is typically regulated to a setpoint value by damper positioning. In one particular prior art arrangement, the hot and cold air dampers are moved in the same direction for flow corrections and they are moved in opposite direction for outlet temperature corrections.
When a feeder speed change occurs in response to a change in fuel demand from the boiler control, the rate at which coal is fed to the pulverizer changes. Then, as the amount of pulverized coal produced for transport by the primary air begins to change, the pressure drop across the mill changes causing the air flow and the exit temperature to change. For example, with a feeder speed increase the amount of pulverized coal begins to increase causing greater pressure drop and reduced primary air flow and reduced outlet temperature. The pulverizer control operates the dampers to make needed flow corrections during the process transient condition.
In the known prior art, pulverizer control configurations have not been as effective as desirable because the response time needed for achieving primary air flow correction has been excessive. Thus, the time for flow correction is so great in many existing installations that undesirable disturbances occur in the boiler outlet steam pressure when fuel changes are required. The primary reason for this is that pulverizer controls typically employ feedback control loops which provide correction only after process errors begin to occur. Thus, the time required for a change in feeder speed to be effected through the pulverizer process significantly affects the response time of prior art pulverizer controls. While feedforward control may generally avoid process error and improve process response, there is no known prior art which discloses how feedforward control can be successfully employed in achieving improved pulverizer control response time without disturbing the smooth and continuous flow of coal to the burners, i.e., without causing settlement of fines and without causing oversize coal to be held in suspension above the pulverizing mechanism.